1. Field of the Invention
The present invention relates generally to applying top tension to a riser extending from a subsea location to a surface vessel used in offshore production of hydrocarbons, the riser accommodating relative movement between the seabed and surface vessel.
2. Description of Related Art
The valve assembly, often known as Christmas tree, which is used to control wells producing hydrocarbons offshore, may either be placed on the seabed or on a surface structure. When the Christmas tree is to be placed at a surface vessel, a production riser is installed between the surface structure and the wellhead. The hydrocarbons are produced through a tubing which is run through, and hung off from the Christmas tree, the production riser and the casing assembly of the well, penetrating into the strata from which hydrocarbons are produced. The production riser acts as a second fluid barrier in the event of leakage from the tubing or the wellhead. In order to run tubing and perform other operations in the well, the risers need to be straight and near vertical. Substantial top tension needs to be applied to maintain straightness and to prevent fatigue due to excessive dynamic response due to the action of waves and current.
This type of risers will buckle and be destroyed if they are subjected to compression. They must therefore be tensioned over their entire length. This tension may be applied with hydraulic or pneumatic cylinders, buoyancy tanks or counterweights. Current practice is to connect the wellheads to the processing equipment on deck with flexible jumpers.
U.S. Pat. No. 5,553,976 proposes an ocean riser assembly including a fluid conveying pipe extending between the seabed and the surface and being formed into a helical configuration or undulating configuration over its entire length, the pipe being attached at spaced intervals to flexible and elastic tension members extending in the longitudinal direction of the riser. Such an arrangement is difficult to install and needs additional buoyancy along its entire length to maintain its structural integrity, such buoyancy adding substantially to the cost of the riser assembly.
WO 01/14687, which belongs to the Assignee of the present application, shows a hybrid riser configuration for use in very deep waters. The hybrid riser comprises a tower structure containing a plurality of steel riser pipes extending vertically from the seabed to a buoyancy tank located about 100 meters below the sea surface. Each riser pipe is confined in an aluminium guide conduit, which acts as a means for mooring the buoyancy tank and which also permits the riser pipe to buckle in Euler deformation into a spiral along the inside of the aluminium guide conduit. This arrangement permits the use of relatively thin-walled steel riser pipes. However, these pipes have to be terminated in the buoyancy tank, where they are connected to so-called jumpers, which are flexible pipelines which lead to the surface vessel and have sufficient slack to permit excursions by the surface vessel. These jumpers hinder the running of tubing and other operations in the well.
Both WO 01/14687 and U.S. Pat. No. 5,553,976 are hereby incorporated by reference.